XHamsterIE: Fix support for new HD video url format and add test

The download of HD videos doesn't work.
The SEO string must be part of the URL or you get redirected to an address with GET-Parameters stripped.
E.g.: http://xhamster.com/movies/2221348/.html?hd --> http://xhamster.com/movies/2221348/britney_spears_sexy_booty.html